Police detained 12 Turk Telecom employees in Istanbul on Tuesday as part of a probe into the Fetullah Terrorist Organization (FETO).

According to Istanbul’s Chief Public Prosecutor's Office, orders to arrest 22 suspects had been issued.

The authorities continued to look for the remaining suspects, sought on charges of "being members of a terrorist organization". The Istanbul-based operation is ongoing in other parts of Turkey, including Agri, Malatya and Ankara provinces.

So far 144 Turk Telekom employees have been taken into custody while several Turkish courts have remanded 63.

The Turk Telecom staff members were detained over suspected links to the FETO group, which is accused of orchestrating Turkey’s defeated July 15 coup attempt which saw 248 people martyred.

They face charges of violating the constitution, treason and membership of a terrorist organization.

ByLock arrests

Meanwhile, at least 20 people were also arrested across eight provinces for allegedly using a smartphone messaging app called ByLock which has been linked to the coup plot.

The operation focused on FETO structures in Turkey’s prison system. Among those detained was the governor of Istanbul's Silivri Penal Institution.

According to Istanbul’s Chief Public Prosecutor's Office, orders to arrest 25 suspects had been issued.

Turkey's government has said FETO, which is led by U.S.-based Fetullah Gulen, is behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police and judiciary.

In the wake of the coup attempt, tens of thousands FETO suspects have been arrested, including many in the armed forces, police, justice system and education sector.

Testimonies given by suspects, including top military figures, video footages and seized documents reveal the mastermind role played by U.S.-based preacher Fetullah Gulen, the top suspect in the ongoing probes into the July 15 coup attempt.

Top general 'told to talk with Gulen'

Turkey's top general said Monday that on the night of the July 15 coup attempt, he was asked to speak on the phone with U.S.-based preacher Fetullah Gulen, the accused mastermind of the coup attempt.

Chief of Staff Gen. Hulusi Akar, testifying as a plaintiff to Ankara prosecutors, said while he was held hostage by pro-coup soldiers, Gen. Hakan Evrim, the commander of a main jet base, asked him to speak with Gulen.

"Hakan Evrim said they could put me on phone with Fetullah Gulen, who he described as their 'opinion leader'," Akar said. "But I refused the offer at once."

"I believe those coup-plotters are members of this organization [the Fetullah Terrorist Organization, or FETO]. I think they thought their organization would take a huge blow after our Supreme Military Council meeting in August -- which we prepared for studiously," Akar said.

"This terrorist organization probably foresaw the outcome of the upcoming meeting and attempted a coup by bombing the parliament building and security offices, killing civilians, attacking their own brothers-in-arms and units with a ferocity and dishonor never seen before."

Akar blasted the coup attempt against the elected government as a "dark stain on Turkey's history," especially at a time when Turkish security forces were getting "successful results against the terrorist organization [PKK]."

"During the night of the attempted coup, I saw that the traitors were demoralized. First the video footage of surrendered traitors on the Bogazici (Bosphorus) Bridge, and then our president's address to the crowd at Ataturk Airport destroyed the hopes of the coup-plotting traitors," he added.

In six pages of testimony, Akar stated that near 9 p.m. (1800GMT) on the failed coup night, Maj. Gen. Mehmet Disli entered his room and said something like, “Sir, the operation has begun. We'll take everyone, the brigades and troops are on their way'.

"At first I couldn’t make any sense of what he said," Akar stated. "He might have said something about planes. But I understood that there was an operation which I can describe as an uprising. I yelled at him: 'What the hell are you talking about! Are you crazy, don't even think about it!'"

Turkey's top general also said he had warned Disli and the coup plotters about the "path they were taking," and urged him to "end the attempt without causing any deaths and not involving anybody else.”

Akar said the three soldiers rushed to his room, held him down, and then partially strangled and handcuffed him.

After threatening to kill him, Akar continued, the soldiers took him to Akinci air jet base, northwest of Ankara, to force him to read and sign the coup manifesto.

Akar said he "never even touched" the manifesto which the coup soldiers wanted him to sign to topple Turkey’s democratically-elected government.

"When they read their manifesto out to me, I listened cynically and without any interest," Akar said.

"Those traitors even called their junta attempt the 'Peace at Home Council'. They said nothing about who belongs to the council," he explained.

Other senior officers who refused to participate were taken hostage, including Akar, who was later rescued from the pro-coup soldiers.

Turkey's government has repeatedly said the deadly coup attempt on July 15, which martyred at least 246 people and injured more than 2,100 others, was organized by followers of Gulen.

Gulen is also accused of a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police, and judiciary, forming what is commonly known as the parallel state.

Top commander aide admits Gulen 'loyalty'

Lt. Col. Levent Turkkan, the aide of Chief of Turkey's General Staff Hulusi Akar, admitted having links to the Fetullah Terrorist Organization, which he said was the main perpetrator of July 15 failed coup attempt.

In his testimony released July 20, Turkkan said he was a loyal member of the FETO group since his youth.

"I am a member of the parallel state, or FETO. I have served this community for years voluntarily. I have obeyed the orders and instructions of the big brothers exactly," he confessed.

Turkkan was detained earlier for his alleged link to the deadly coup attempt on July 15, which martyred at least 246 people and injured more than 2,100 others.

Turkkan said he came from a poor family in the northwestern province of Bursa and met the group during his secondary school years while he was staying at a dorm.

"Since I was five, my biggest wish was to become a military officer. [...] And my dream of becoming a military officer pleased them," he said, adding that he entered the military school entrance exam in 1989.

Turkkan said he was a bright student and that he was sure he would pass the exams with his own efforts.

"But the night before the exam, big brothers gave me the [answered] questions of the exam in a house belonging to them in Bursa," he said.

Continuing to see his "big brothers" during and after his military school years, Turkkan said he "served them during missions in Istanbul, Trabzon, Diyarbakir, Lefkosa, Kiziltepe and lastly in Ankara."

Turkkan also confessed he spied on former Chief of Staff Necdet Ozel during his post between 2011- 2015. Turkkan said he initially served as deputy aide and later became an aide after his chief retired.

Turkkan said he fulfilled the Gulen movement’s orders after becoming a close aide in the General Staff.

"I was spying on [former] Chief of Staff Necdet Ozel all the time. I put a recording device in the room in the morning and took it back in the evening hours," Turkkan said. "The device had its own capacity and could wiretap for 10-15 hours."

During the questioning by the prosecutors, Turkkan admitted he received the device by a person who claimed to be working at Turk Telekom, Turkey's telecommunications center.

“[He] ordered me to eavesdrop on the general [Necdet Ozel]. He told me ‘We will tap him for intelligence purposes; nothing is going to happen.’ I did not question and took the device,” he said.

Turkkan said he had at least two recording devices and returned the devices once their capacity was full.

Lt. Col. Turkkan also admitted that Major Mehmet Akkurt -- whom Turkkan claimed to be a member of Gulen movement and worked closely with -- spied on high-ranking generals, including the Chief of Staff Hulusi Akar.

He did not specify the exact time of when they tapped Akar.

“I think the movement has been spying on those generals to know what is going on inside the armed forces,” Turkkan confessed.

Once General Hulusi Akar was promoted as the Chief of Staff, Turkkan said he stopped wire-tapping.

During the interrogation, Turkkan also provided information on how the Gulen movement was organized inside the Turkish military.

“I believe 60-70 percent of those people who have been accepted inside the armed forces since 1990s are Gulen-linked people,” he said.

Turkkan said he received information about the military coup on July 14 at around 10 a.m. local time (1900GMT) from Staff Colonel Orhan Yikilkan, who served as an adviser to the chief of staff.

"Yikilkan told me the president, the prime minister, the chief of staff and commanders-in-chief would be arrested; that would be done quietly; [the military coup] would be staged at 3 a.m. on early Saturday [July 16],” he said.

Turkkan said he visited his brother to check whether he was aware of a military take-over, but failed to see him.

He told the prosecution he met with other members of the movement in his brother’s home.

“When I asked them, they angrily told me ‘How do you know? Who told you these? Did you tell anyone?’… They told me to keep quiet,” Turkkan said.

After the thwarted attempt, Turkkan said he surrendered to the military officials who turned him in to the police.

Turkkan said he regretted taking part in the violent coup.

"Until the coup, I thought Gulen movement was acting for only God's sake and Gulen himself had a spiritual identity," he said.

"Until today, I never thought Gulen movement was a traitor. But now I realize what they really are. They [the members of FETO] are bloodthirsty. I have never seen Fetullah Gulen, but he is the same," he said.

“I feel regret not only for taking a part in this but also becoming a member of the Fetullah Gulen movement,” he added.

Police caught in military uniform

The first concrete evidence that FETO was behind the coup attempt was uncovered on the night of the attempt, when Istanbul's former Security Branch Manager Mithat Aynaci, was detained after being caught in a tank dressed in military uniform.

Aynaci had been suspended from duty in November 2014 over suspected links to FETO. He was later dismissed.

Alleged chief of assassination team

The alleged chief, or the so-called “imam” of the SAT commandos who were planning to assassinate government figures, said in his testimony “the big brother” role was offered to him during talks with FETO members, and that was how he started to communicate with the commandos.

Kemal Isikli, an expert at the Banking Regulation and Supervision Agency, said he was first introduced to FETO in 1997, when he attended the Sabah Private Tuition Center in the central province of Konya.

"During one of the talks, someone whose name escapes me now, asked me to be a ‘big brother’,” Isikli said, adding that afterwards, six non-commissioned officers who were SAT staff started coming over to his place, where they recited the Quran, performed prayers, and talked.

“We never discussed politics,” he said, adding he became a part of the FETO structure in 2011.

‘Gendarme imam’ confesses

Hasan Bektas, one of FETO’s so-called “gendarme imams in charge of non-commissioned gendarmes”, said in his testimony that FETO was one of the biggest crime organizations.

Bektas said he was introduced to FETO in 1998, when he was in tenth grade, and attended one of the private tuition centers run by the movement.

He said he lived in one of the apartments belonging to FETO during his university education in Konya, and later on started teaching at the organization’s schools.

Brig. Gen. admits FETO link

Brig. Gen. Fatih Celaleddin Sagir is another army general which admitted he was linked to FETO.

Commander of the 5th Infantry Training Brigade in central Sivas province said in his testimony he was introduced to FETO during his years at the Military Academy.

“I was in contact with them from 1988 to 1992. However, from 1992 until 2007, I was just a sympathizer,” he said.

Sagir was alleged to be promoted as a commander of martial law in case the coup attempt succeeded.

Turkey's government has repeatedly said the deadly coup attempt on July 15 was organized by followers of Gulen.

Gulen is also accused of running a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police, and judiciary, forming what is commonly known as the parallel state.